The present invention relates to a method of operating a hydrostatic or pneumatic drive for a mass which is to be accelerated and decelerated, and also to such a hydrostatic or pneumatic drive.
In a drive of that kind, the drive arrangement includes a motor such as a linear or rotary motor, a fluid pressure source for supplying a hydraulic or pneumatic pressure fluid, and a valve arrangement. The valve arrangement has a restricted passageway which can be controlled in order to produce a variable working flow which is subject to fluctuating working pressures, thereby giving a compression volume in respect to the pressure fluid, relative to an initial condition of the arrangement depending on the working pressure. The arrangement also includes a reference value control generator for controlling the valve arrangement.
DE-A-specification No. 2 902 264 discloses a control device for a hydraulic installation, wherein defined acceleration or retardation of the drive arrangement can be produced by a defined increase or decrease in the flow of pressure fluid. That arrangement includes various electronic control means, for avoiding sudden jerks in acceleration. More specifically, disposed between the pump and the motor of the arrangement is a four-way directional valve having three operative positions, the valve being electrically operable, with ramp generating means being included in the electrical control arrangement. The output signal of the ramp generating means is applied to an electrical switching amplifier which is then operative to control the directional control valve. That arrangement does not include any provision for making allowances for the fact that the hydraulic fluid is compressible and that the components of the drive arrangement are caused to expand upon an increase in pressure.
DE-A-specification No. 28 08 694 discloses another positioning means for a hydraulic drive, which seeks to avoid a sudden change in speed of the drive. For that reason, a ramp generating means is disposed between a reference value control generator and an actual value detector means on the one hand, and an electrically actuable hydraulic control member on the other hand. This arrangement also takes no account of the compression volume of the system.
In regard to injection moulding machines. FR-A-specification No. 22 99 103 discloses that an injection moulding mould is to be filled more slowly, towards the end of the filling stroke movement of the system, because otherwise there would be a pressure peak. Therefore, before the mould is in the completely filled condition, the speed of the injection piston of the injection moulding machine is reduced thereby also to reduce the speed of filling of the mould, as it approaches the completely filled state.
Still dealing with die casting and injection moulding machines, it will be appreciated that a mass which is at rest or which is being moved is to be driven by means of a piston cylinder unit as the motor of the drive so that the mass can be caused to move at a given speed. For that purpose, the valve arrangement is opened to a gradually increasing extent, in order thereby to apply an increasing pressure to the piston which is operative to drive the mass in question. As referred to above however, the hydraulic fluid is not incompressible, while the components of the drive may also tend to suffer from an expansion or swelling effect, more particularly the hydraulic lines of the arrangement, with the result that a certain proportion of the flow of hydraulic fluid supplied to the system is absorbed in a capacitive-like manner by the drive system, and does not directly result in displacement of the mass to be driven by the drive system. It will be seen furthermore that the capacitively absorbed pressure fluid represents a stressed spring which, with the mass to be moved, represents an oscillatory system. The stimulus for oscillation of that system increases in proportion to the rate at which the drive arrangement and the mass connected thereto are to be brought up to the desired speed of movement. Therefore, when the electrical control arrangement has the above-mentioned reference value control generator for controlling the operation of the drive, if the generator produces a steep ramp voltage, the system responds to that mode of operation with weakly damped oscillations; in other words, the pressure only gradually settles down to a desired reference value, with the result that the piston of the drive arrangement only gradually attains the desired, steady speed. In practice therefore, a ramp voltage having a ramp time in the order of magnitude of 400 ms is used. In this respect, experts were of the opinion that the cycle time of injection moulding machines, which involves such ramp times cannot be reduced to any substantial extent (see `Microelectronics in Injection Molding Machines` by Dr. W. Elbe and R. K. Jackson, from Mannesmann-Demag and Mannesmann-Demag-Hamilton, in particular page 4 thereof).